I don't see how it's a free kill.
because he's sitting sitting still.
how about this "solution"? if a vehicle gets killed by a bomb and is not in motion, have it count simply as an object destroyed instead of a kill!
i dont see a problem either way.. if you are bombing gv's at an enemy base, you're probably trying to capture it.. put your bombs on the vh(s), then on the gv's.. if they tower out then, who cares, they're out of the fight either way, you just don't get an easy kill to go along with it.
also, i believe hitech has addressed the issue, and said it is like a defender's homefield advantage.